Wilms tumor (nephroblastoma) is a pediatric kidney cancer that arises in renal precursor cells and constitutes a prime example of the link between normal organ development and tumorigenesis. The WT1 tumor suppressor gene encodes a zinc finger transcription factor, thought to be a master regulator of kidney differentiation, whose inactivation in a subset of Wilms tumors triggers malignant proliferation. We have characterized the two primary WT1 splicing variants (+/-KTS): WT1(-KTS) encodes a transcriptional regulator of a cellular differentiation program, but the function of the equally essential WT1(+KTS) isoform is unknown. Using a chromatin immunoprecipitation strategy, we have recently uncovered potential targets of WT1(+KTS) and now propose to characterize their regulation by WT1 and their role in normal renal differentiation and tumorigenesis. We have also studied additional transcriptional regulators that affect renal differentiation, including the Forkhead transcription factor BF2/FoxD, whose expression by stromal cells is required for the differentiation of neighboring epithelial cells, identifying the secreted Placenta! Growth Factor as a transcriptional target of BF2. We have also studied Polycomb group genes (PcG), which are essential for maintaining the repressed state of Hox genes, using the single C. elegans ortholog SOP-2 to define key functional elements that are highly conserved across evolution. To search for additional Wilms tumor genes, we have used array comparative genomic hybridization to identify novel sites of chromsomal gains and losses in primary Wilms tumor specimens. We have identified two novel candidate genes that are targeted by intragenic mutations in a significant number of cases. We propose genetic and functional analyses of these new candidate Wilms tumor genes. Relevance of Research Project to Public Health: Wilms tumor is the most common kidney cancer in children, and while many cases are now curable, a significant number of children still die of this tumor. An understanding of the genetic causes of Wilms tumor is now possible and may lead to improved classification and subtype-specific treatment for differnt forms of this cancer. In addition, Wilms tumor is an invaluable model for other embryonal tumors of childhood, and defining the relevant mechanisms that govern its proliferation will have important impact for other human tumors that originate in such precursor cells.